What's the difference between nauseous and nauseated?

Nausea, by itself, is the urge to vomit.

I had really bad nausea.

Nauseated is the verb meaning to become affected with nausea.

I felt really nauseated all of a sudden.

Nauseating is the quality of inflicting nausea on someone.

Man, that smell was really nauseating.

Nauseous is the weird one, which can mean either 'nauseated' or 'nauseating'. Some folks definitely believe that its only proper use is 'nauseating', but Merriam Webster's usage notes disagree:

Those who insist that nauseous can properly be used only in sense 1 ["causing nausea or disgust : NAUSEATING"] and that in sense 2 ["affected with nausea or disgust"] it is an error for nauseated are mistaken. Current evidence shows these facts: nauseous is most frequently used to mean physically affected with nausea, usu. after a linking verb such as feel or become; figurative use is quite a bit less frequent. Use of nauseous in sense 1 is much more often figurative than literal, and this use appears to be losing ground to nauseating. Nauseated is used more widely than nauseous in sense 2.

The story of 'nauseous' as told by Merriam-Webster's Collegiate Dictionaries

The history of nauseous in Merriam-Webster's Collegiate Dictionary series is quite interesting. As late as the Sixth Collegiate (1949), MW's entry for nauseous has just one definition:

nauseous adj. Causing, or fitted to cause, nausea; disgusting.

The Sixth Collegiate was MW's Collegiate Dictionary of record until 1963 (my copy is copyrighted 1959). But the Seventh Collegiate (1963) contains a radically altered the entry for nauseous:

nauseous adj. 1 : NAUSEATED 2 : causing nausea : SICKENING

Not only does nauseous as "nauseated" make its debut in the Seventh Collegiate, but it occupies the first (that is, chronologically earlier) meaning of the word, bumping nauseous as "nauseating" to the second slot in the entry.

In the Eighth Collegiate (1973), Merriam-Webster reverses its predecessor's judgment about which meaning has historical priority, shifting nauseous as "nauseating" to the first (earlier) position:

nauseous adj 1 : causing nausea : SICKENING 2 : affected with nausea or disgust

The other significant change in the Eighth Collegiate's treatment of nauseous is its replacing of the one-word definition "NAUSEATED" with a five-word descriptive definition. Though the Seventh Collegiate equated nauseous in that sense with nauseated, it didn't define the latter word; instead readers were left to surmise the meaning of nauseated from the dictionary's three-definition entry for nauseate:

nauseate vt 1 : to become affected with nausea 2 : to feel disgust ~ vi : to affect with nausea or disgust

In the absence of a separate entry for nauseated—and given that the verb nauseate had the past-tense form nauseated for all three meanings given in the entry for nauseate—readers were left at sea with the Seventh Collegiate's one-word primary definition of nauseous; it's easy to see why Merriam-Webster saw the need to improve on that definition for the Eighth Collegiate.

One point that frequently goes unnoticed in discussions of nauseous is that, all along, the verb nauseate could be understood in both the "cause nausea" sense and the "feel nausea" sense. The Sixth Collegiate (for example) has this entry for the verb:

nauseate v.t. & i. To affect or become affected with nausea; disgust; sicken.

The Ninth Collegiate (1983)—the edition that introduced the first-occurrence date feature in its word entries—tinkered with the first definition of nauseous, but not with the second:

nauseous adj (1612) 1 : causing nausea or disgust : NAUSEATING 2 : affected with nausea or disgust

The Ninth Collegiate also marks the appearance of the first version of the usage note that Lynn quotes in another answer to this question. Here is how MW words the original usage note:

usage Those who insist that nauseous can properly be used only in sense 1 are in error. Current evidence shows these facts: nauseous is most frequently used to mean physically affected with nausea; extended use is quite a bit less frequent. Use of nauseous in sense 1 is much more often figurative than literal, and this use appears to be losing ground to nauseating. There seems to be little use of nauseated except by those who prescribe it in place of nauseous.

In the Tenth Collegiate (1993) the definitions of nauseous are identical to those in the Ninth Collegiate. The usage note, however, shows some significant alterations (rendered in bold below):

usage Those who insist that nauseous can properly be used only in sense 1 and that in sense 2 it is an error for nauseated are mistaken. Current evidence shows these facts: nauseous is most frequently used to mean physically affected with nausea, usu. after a linking verb such as feel or become; figurative use is quite a bit less frequent. Use of nauseous in sense 1 is much more often figurative than literal, and this use appears to be losing ground to nauseating. Nauseated, while not rare, is less common than nauseous in sense 2.

Like the Tenth Collegiate, the Eleventh Collegiate (2003) retains the Ninth Collegiate's definitions unchanged. But it, too, substantially revises the usage note. In this case the few but crucial changes occur in the final sentence of the note (again rendered in bold below):

usage Those who insist that nauseous can properly be used only in sense 1 and that in sense 2 it is an error for nauseated are mistaken. Current evidence shows these facts: nauseous is most frequently used to mean physically affected with nausea, usu. after a linking verb such as feel or become; figurative use is quite a bit less frequent. Use of nauseous in sense 1 is much more often figurative than literal, and this use appears to be losing ground to nauseating. Nauseated is used more widely than nauseous in sense 2.

Over a period of about 65 years, Merriam-Webster's Collegiate Dictionary series doesn't just record (in the Seventh Collegiate) a shift from nauseous defined as having only the meaning "Causing, or fitted to cause, nausea; disgusting" to nauseous having two meanings: "causing nausea or disgust : NAUSEATING" and "affected with nausea or disgust." It also records (beginning in the Ninth Collegiate) the predictable demise of nauseous in the sense of "nauseating" in favor of the unambiguous word nauseating; and perhaps more surprisingly, it rather grudgingly records (over the span of the Ninth, Tenth, an Eleventh Collegiates) the rise of the word nauseated as a seemingly unambiguous alternative to nauseous in the sense of "affected with nausea or disgust."

On this record, it appears that the emergence of nauseous as "nauseated" in a sphere previously occupied solely by nauseous as "nauseating" led to a significant level of user flight away from the use of nauseous to indicate either sense of the word, in favor of words (nauseating and nauseated) that were perceived to be completely unambiguous. Whether nauseated is in fact technically unambiguous is debatable, but in popular usage I've never heard anyone say, for example, "The food was so nauseated that I couldn't eat it."

What the Ngram results say

An Ngram chart over the years 1800–2005 for nauseate (blue line) nauseated (red line), nauseating (green line), and nauseous (yellow line) tells a somewhat different story:

Most notable here is the steady but cumulatively severe decline in the frequency of nauseous from 1800 or so until about 1940, followed by a flattening out over the next 40 years and a slight rise since 1980. Tightly bunched with nauseous in the results for the period from 1920 to 2005 are nauseating and nauseated, while nauseate remains far less common than the other three words.

The most striking thing about the Ngram results for 1949–2003 is how flat everything generally is:

It's rather astonishing that during this period (in the United States) a revolution in the accepted meaning of nauseous occurred. Looking at the portion of the chart representing those years, an observer would probably conclude that nothing much had happened to nauseous during most of that interval of time.

Conclusions

Today nauseous has two meanings: the old one ("sickening") that many people have stopped using nauseous to signify, preferring instead to use the unambiguous word nauseating; and the newer one ("made sick") that many people do use nauseous to signify but that others now prefer to use the effectively unambiguous word nauseated to signify. The history of nauseous thus illustrates how language sometimes adapts, shifts, or reverses the meaning of longtime words, and how speakers and writers sometimes respond to ambiguity with further changes in word choice and usage.